Permanent magnet rotor and method of making the same

Details Permanent magnet rotor and method of making the same Permanent magnet rotor and method of making the same

2001-06-15

2003-10-07

Nguyen, Tran (Department: 2834)

Electrical generator or motor structure

Dynamoelectric

Rotary

C310S156560

Reexamination Certificate

active

06630762

ABSTRACT:

RELATED APPLICATIONS
The present application claims the benefit of priority under 35 U.S.C. §119(a) from Japanese Patent Application No. 2000-181239, filed on Jun. 16, 2000, which is incorporated by reference herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a permanent magnet rotor used in a rotary electric device (including a device that has a rotating rotor and a device that has a rotating coil), such as, for example, a permanent magnet electric motor and a permanent magnet generator. This invention also relates to a method of making such a rotor. This invention particularly relates to a permanent magnet rotor having an embedded magnet. The rotor provides effective utilization of the reluctance torque and reduces the leakage flux caused by the magnetic flux of the permanent magnets passing through bridges in the rotor to form a loop instead of flowing into the stator.
2. Description of the Related Art
A conventional embedded magnet type permanent magnet rotor is disclosed, for example, in Laid Open Japanese Patent Application Hei 11-262205 or in Laid Open Japanese Patent Application Hei 11-206075. The shape of one pole (in end view) of a permanent magnet rotor core
1
of an exemplary rotor
10
is illustrated in FIG.
7
. As illustrated, the rotor core
1
is formed with a plurality of slits
2
A,
2
B,
2
C in multiple layers. Each of the slits
2
A,
2
B,
2
C has an end face in the shape of an arc. Each arc is configured such that the longitudinal ends of the arc are located in the vicinity of the outside circumferential surface of the rotor core
1
and such that the longitudinal middle portion of the arc is located radially inwardly of the end portions. Each slit extends to the opposite end of the rotor core
1
in the axial direction (perpendicular to the plane of
FIG. 7
) with the same cross-sectional shape as the shape of the end face shown in FIG.
7
.
In order to form a permanent magnet rotor
10
with a rotor core
1
having embedded permanent magnets, the slits
2
A,
2
B,
2
C are filled with a bond magnetic material (e.g., a plastic magnetic material) by injection molding, or the like. After filling the slits, the bond material is solidified. Alternatively, a respective permanent magnet may be machined in the shapes of each of the slits
2
A,
2
B,
2
C, and the machined magnets are then fitted into the corresponding slits
2
A,
2
B,
2
C.
Furthermore, with respect to the permanent magnet rotor
10
shown in
FIG. 7
, bridges
3
of a certain thickness are formed between the longitudinal ends of the slits
2
A,
2
B,
2
C and the outside circumferential surface of the core
1
so that the radially outer portions (i.e., the portions on the outside circumferential surface side) and the radially inner portions (i.e., the portions on the center axis side) of the rotor core
1
with respect to the respective slits
2
A,
2
B,
2
C will not be completely separated by the slits
2
A,
2
B,
2
C.
However, it has been shown that in the foregoing conventional permanent magnet rotor
10
of
FIG. 7
, the bridges
3
formed in the rotor core
1
at the outside circumferential surface side ends of the slits
2
A,
2
B,
2
C allow leakage flux to flow through the bridges
3
, which prevents effective use of the permanent magnets. For example,
FIG. 8
shows the magnetic flux generated in the permanent magnet rotor
10
and the stator pole teeth
20
, in dashed lines, and illustrates the leakage flux SF flowing through the bridges
3
.
In addition, the leakage flux SF in the bridges
3
causes portions of the magnetic flux density to be greater in the area of the bridges than in the surrounding areas, as illustrated in
FIG. 8
by a symbol A. This causes a magnetic resistance in the magnetic paths of the q-axis magnetic flux &PHgr;q to increase, which is a factor of lowered reluctance torque.
Torque generated by the motor is written as:
T=
(
Pn×&PSgr;a×iq
)+(
Pn×
(
Ld−Lq
)×
id×iq
)  (1),
where:
Ld is the d-axis inductance of the coil,
Lq is the q-axis inductance of the coil,
id is the d-axis component of the armature current,
iq is the q-axis component of the armature current,
&PSgr;a is the interlinking flux of the armature coil due to permanent magnets, and
Pn is the number of the pairs of poles.
The direction of the d-axis is a direction of a line connecting the center of the magnet poles and the center of the rotor. The direction of the q-axis is a direction of a line passing between the poles and through the center of the rotor. That is, the direction of the q-axis is a direction at 90 degrees in electrical angle with respect to direction of the d-axis.
The first term of the expression (1) represents a torque due to the permanent magnets, and the second term a reluctance torque.
FIGS. 9 and 10
illustrate the conventional permanent rotor
10
in end view, depicting the directions of the q-axis and the d-axis. The dashed lines in
FIG. 9
illustrate the directions of the q-axis magnetic flux &PHgr;q(=Lq×iq) generated by iq. The dashed lines in
FIG. 10
illustrate the d-axis magnetic flux &PHgr;q(=Ld×id) generated by id.
In the embedded magnet type permanent magnet rotor, permanent magnets magnetically equivalent to air gaps are disposed in the magnetic paths of the d-axis magnetic flux &PHgr;d, so that the d-axis inductance Ld is small. On the contrary, the magnetic paths of the q-axis magnetic flux &PHgr;q pass through the rotor core
1
, so that the q-axis inductance Lq is large (that is, the magnetic resistance is small). Therefore, Ld<Lq, and appropriate currents id, iq will generate the reluctance torque (Ld−Lq)×id×iq.
Portions with high magnetic flux density in the bridges
3
narrow the magnetic paths of &PHgr;q and increase the magnetic resistance of the q-axis magnetic paths, which constitutes a factor of lowering the reluctance torque.
SUMMARY OF THE INVENTION
In view of the foregoing unsolved problem with the conventional permanent magnet rotor, embodiments of the present invention are directed to a permanent magnet rotor capable of providing an effective utilization of embedded magnets and the reluctance torque and to a preferred method of making the such a rotor.
One aspect of the present invention is a permanent magnet rotor with a rotor core that has permanent magnets embedded therein in slits. The longitudinal ends of the slits in which the permanent magnets are embedded are open to the outside in the circumferential surface of said rotor core. Bridges that connect the radially outer portions and the radially inner portions of the rotor core with respect to the respective slits are provided at positions inwardly of the longitudinal ends of the slits toward the longitudinal middle portions.
In preferred embodiments of this aspect of the invention, the bridges are formed, not at the longitudinal ends of the slits, but at positions inwardly of the longitudinal ends toward the middle portions of the slits. Therefore, the leakage flux is produced from either end of the bridges, so that the region of high magnetic density narrowing the magnetic paths between slits, is decreased with the help of the leakage flux. As a result, increased magnetic resistance in the magnetic paths of the q-axis magnetic flux &PHgr;q is prevented and the reluctance torque can be utilized effectively.
In preferred embodiments in accordance with this aspect of the present invention, the permanent magnets in the permanent magnet rotor are formed such that the slots are filled with bond magnet (i.e., plastic magnetic material) and the bond magnet is then solidified.
Preferably, the bond magnet is used to form the permanent magnets by injection molding, so that the permanent magnets can be embedded in the rotor core even when the shape of the slits is rather complicated. The injection molding using bond magnet may be an ordinary one in which the bond magnet is filled in the slits and solidified. On the other hand, if the bond magnet is

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